In general, a microcomputer is employed to control a variety of operations of an automobile, and a stepping motor may be used as a controlled output. Among the functions of the microcomputer, it is used to diagnose abnormal conditions in control systems, and upon detection of an abnormal condition, the microcomputer functions to return the control system to a safe state. This function is essential for the security of an automobile.
FIG. 5 shows one example of such a control system, namely, a control circuit for a vehicle speed responding device of a power steering device. In FIG. 5, a vehicle speed sensor 1 applies a pulse signal having a frequency corresponding to the vehicle speed as detected from the transmission system, as a control input signal to a microcomputer 2. The microcomputer 2 has output terminals OA.sub.0, OA.sub.1, OA.sub.2 and OA.sub.3, and applies a digital signal corresponding to the vehicle speed through the respective output terminal to a driver circuit 3. The driver circuit 3 is adapted to switch on and off the circuits of phase coils A, B, C and D of a stepping motor 4. The stepping motor 4 is a four-phase motor and can take eight (8) rotational positions by selectively applying current to the coils A, B, C and D. As the rotational position changes, the degree of opening of a flow control valve is changed, and the quantity of oil supplied to the power steering device is changed accordingly. That is, aspects of the power steering operation are carried out according to the vehicle speed.
The microcomputer 2 has input terminals IB.sub.0, IB.sub.1, IB.sub.2 and IB.sub.3 for self-diagnosing the control system. The input terminals are connected to the output terminals of the driver circuit.
For instance, when the microcomputer 2 applies an output signal (1 0 0 1) to the output terminals OA.sub.0 through OA.sub.3 to turn on transistors 3.sub.0 and 3.sub.3 turn off transistors 3.sub.1 and 3.sub.2, and allow current to flow in the coils A and D. A signal (0 1 1 0) is accordingly supplied to the input terminals IB.sub.0 through IB.sub.3. In this case, the microcomputer 2 compares the output signal (1 0 0 1) with the input signal (0 1 1 0), thus determining that the control system is operating normally.
If, when the output signal is (a.sub.0, a.sub.1, a.sub.2, a.sub.3), the input signal is not (a.sub.0, a.sub.1, a.sub.2, a.sub.3), the microcomputer applies a "0" signal to an output terminal OB.sub.0 to turn off the transistor 5 which is inserted in the common power source circuit of the coils A, B, C and D, to deenergize the stepping motor 4. The rotational position of the stepping motor is then set back to the zero position by the elastic force of a spring (not shown), so that the flow rate of oil is made minimum and the power assitance of the power steering is made minimum, i.e., a safe state.
FIG. 6 shows the driver circuit 3 in more detail. Both ends of the coil A connected to the transistor 3.sub.0 are connected to a protective circuit including a diode 8.sub.0. The input terminal of a buffer 11.sub.0 connected to the input terminal IB.sub.0 of the microcomputer 2 is connected through a diode 9.sub.0 to the connecting point of the coil A and the transistor 3.sub.0. A forward bias is applied through a resistor 10.sub.0 to the diode 9.sub.0. The remaining transistors 3.sub.1, 3.sub.2 and 3.sub.3 are also connected in the same manner.
In the above-described control system, it is impossible to self-diagnose the breaking of any of the coils A, B, C and D of the stepping motor 4. For instance in the case where the coil A has been broken, when the logic signal "1" is applied to the output terminal OA.sub.0, current flows through the resistor 10.sub.0 and the diode 9.sub.0 to the transistor 3.sub.0 which is turned on, and a signal "0" is applied to the input terminal IB.sub.0. That is, when the logic signal (a.sub.0) is provided at the output terminal OA.sub.0, the logic signal (a.sub.0) is applied to the input terminal IB.sub.0. Therefore, the coil A is erroneously determined to be normal, i.e., the breaking of the coil A is overlooked.